Photoacoustic trace gas sensing using a miniature 3D printed gas cell

Bauer, Ralf and Stewart, George and Johnstone, Walter and Lengden, Michael (2014) Photoacoustic trace gas sensing using a miniature 3D printed gas cell. In: Photon14, 2014-09-01 - 2014-09-04, Imperial College London.

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Photoacoustic spectroscopy (PAS) as a measurement method for trace gas detection has the inherent property of favourable signal levels with reduction of the gas cell diameter. Approaches in miniaturisation have so far been presented through miniature milling or waver bonding, with the overall technique being employed for leak detection in industrial applications or diagnostics in medical and biological applications. We will present a further miniaturisation approach by using a 3D printed miniature PAS gas cell in combination with off-the-shelf fibre optics and MEMS membrane microphones. To achieve a low cost, small scale system, rapid prototyping fabrication is used with a stereolithography 3D printer (EnvisionTec Prefactory Desktop Aureus), building the 3D printed gas cell in one piece using laser induced polymerisation of an acrylic resin. The resonant PAS gas cell has a cylindrical acoustic resonator of 9.86mm length and 0.9mm radius, acoustic buffer volumes connected at the end of the open resonator and integrated holders for two fibre coupled gradient index (GRIN) lens collimators creating the optical interrogation path. Multiple miniature MEMS membrane microphones (Wolfson Microelectronics WM7131) are connected to the middle of the resonator to detect the pressure changes induced by modulation of the interrogating laser source at the first longitudinal acoustic resonance of the cell, therefore, amplifying the signal by the Q-factor of the resonant cell. The overall outer dimensions of the gas cell are less than 15mm x 25mm x 29mm. We present our latest results on detection limits of the 3D printed gas sensor, using fibre coupled DFB lasers for interrogation of gas species in the NIR and quantitative measurements with calibrated gas mixtures. A further focus will be on integration of amplitude modulation techniques using, for example, a semiconductor optical amplifier and additional steps to increase signal sensitivities and aid system integration.